A system for identifying microscopic particles in a fluid such as water, includes a light source such as a laser that directs light through a small detect zone, while particles in the fluid occasionally pass through the detect zone. When a particle passes through the detect zone, it scatters light. One or more photodetectors detect light scattered in a particular direction, and the scattering pattern can be used to detect and/or identify the particle. Each photodetector or group of them, has an alignment surface with an axis, such as the outside of a tube that encloses lenses and a photocell, that should be precisely aligned with the axis of the field of view. The field of view should be neither greater nor smaller than expected.
The detect zone that one or many photodetectors view, may be small, such as with a width and length each of 1.5 mm. It is important that when the alignment surface axis points at the detect zone, that a photodetector view substantially only the detect zone to detect light scattered from a particle passing through the zone. If the photodetector should detect light from outside the zone or not detect light scattered towards it from the zone, then the system would generate false readings. In one system, each of numerous photodetectors have a field of view that is only about 1.5°±0.5°. A method and apparatus that enabled a precise determination of the actual field of view of a photodetector, and the angle (if any) between the axis of the field of view and the axis of an alignment surface, in a simple and rapid manner, would be of value.